Borehole Conductivity Simulator Verification and Transverse Coil Balancing

ABSTRACT

Calibration of the arrays of a multicomponent induction logging tool is achieved by positioning the tool horizontally above ground. The upper and lower housings of the tool are connected by a borehole conductivity simulator which as a resistance comparable to that of a borehole. Axial and radial positioning of the transmitter coils is done by monitoring outputs at receiver coils to achieve a minimum.

CROSS REFERENCE TO RELATED APPLICATIONS

The present invention is a continuation-in-part of U.S. patentapplication Ser. No. 11/371,052 filed on the 8 Mar. 2006, and U.S.application Ser. No. 11/340,785 filed on 26 Jan. 2006.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is related to the field of apparatus design in thefield of oil exploration. In particular, the present invention describesa method for calibrating multicomponent logging devices used fordetecting the presence of oil in boreholes penetrating a geologicalformation.

2. Description of the Related Art

Electromagnetic induction resistivity well logging instruments are wellknown in the art. Electromagnetic induction resistivity well logginginstruments are used to determine the electrical conductivity, and itsconverse, resistivity, of earth formations penetrated by a borehole.Formation conductivity has been determined based on results of measuringthe magnetic field of eddy currents that the instrument induces in theformation adjoining the borehole. The electrical conductivity is usedfor, among other reasons, inferring the fluid content of the earthformations. Typically, lower conductivity (higher resistivity) isassociated with hydrocarbon-bearing earth formations. The physicalprinciples of electromagnetic induction well logging are well described,for example, in, J. H. Moran and K. S. Kunz, Basic Theory of InductionLogging and Application to Study of Two-Coil Sondes, Geophysics, vol.27, No. 6, part 1, pp. 829-858, Society of Exploration Geophysicists,December 1962. Many improvements and modifications to electromagneticinduction resistivity instruments described in the Moran and Kunzreference, supra, have been devised, some of which are described, forexample, in U.S. Pat. No. 4,837,517 to Barber, in U.S. Pat. No.5,157,605 to Chandler et al., and in U.S. Pat. No. 5,600,246 to Faniniet al.

The conventional geophysical induction resistivity well logging tool isa probe suitable for lowering into the borehole and it comprises asensor section containing a transmitter and receiver and other,primarily electrical, equipment for measuring data to infer the physicalparameters that characterize the formation. The sensor section, ormandrel, comprises induction transmitters and receivers positioned alongthe instrument axis, arranged in the order according to particularinstrument or tool specifications and oriented parallel with theborehole axis. The electrical equipment generates an electrical voltageto be further applied to a transmitter induction coil, conditionssignals coming from receiver induction coils, processes the acquiredinformation, stores or by means of telemetry sending the data to theearth surface through a wire line cable used to lower the tool into theborehole.

In general, when using a conventional induction logging tool withtransmitters and receivers (induction coils) oriented only along theborehole axis, the hydrocarbon-bearing zones are difficult to detectwhen they occur in multi-layered or laminated reservoirs. Thesereservoirs usually consist of thin alternating layers of shale and sandand, oftentimes, the layers are so thin that due to the insufficientresolution of the conventional logging tool they cannot be detectedindividually. In this case the average conductivity of the formation isevaluated.

Conventional induction well logging techniques employ coils wound on aninsulating mandrel. One or more transmitter coils are energized by analternating current. The oscillating magnetic field produced by thisarrangement results in the induction of currents in the formations whichare nearly proportional to the conductivity of the formations. Thesecurrents, in turn, contribute to the voltage induced in one or morereceiver coils. By selecting only the voltage component which is inphase with the transmitter current, a signal is obtained that isapproximately proportional to the formation conductivity. Inconventional induction logging apparatus, the basic transmitter coil andreceiver coil has axes which are aligned with the longitudinal axis ofthe well logging device. (For simplicity of explanation, it will beassumed that the borehole axis is aligned with the axis of the loggingdevice, and that these are both in the vertical direction. Also singlecoils will subsequently be referred to without regard for focusing coilsor the like.) This arrangement tends to induce secondary current loopsin the formations that are concentric with the vertically orientedtransmitting and receiving coils. The resultant conductivitymeasurements are indicative of the horizontal conductivity (orresistivity) of the surrounding formations. There are, however, variousformations encountered in well logging which have a conductivity that isanisotropic. Anisotropy results from the manner in which formation bedswere deposited by nature. For example, “uniaxial anisotropy” ischaracterized by a difference between the horizontal conductivity, in aplane parallel to the bedding plane, and the vertical conductivity, in adirection perpendicular to the bedding plane. When there is no beddingdip, horizontal resistivity can be considered to be in the planeperpendicular to the bore hole, and the vertical resistivity in thedirection parallel to the bore hole. Conventional induction loggingdevices, which tend to be sensitive only to the horizontal conductivityof the formations, do not provide a measure of vertical conductivity orof anisotropy. Techniques have been developed to determine formationanisotropy. See, e.g. U.S. Pat. No. 4,302,722 to Gianzero et al.Transverse anisotropy often occurs such that variations in resistivityoccur in the azimuthal direction.

Thus, in a vertical borehole, a conventional induction logging tool withtransmitters and receivers (induction coils) oriented only along theborehole axis responds to the average horizontal conductivity thatcombines the conductivity of both sand and shale. These average readingsare usually dominated by the relatively higher conductivity of the shalelayers and exhibit reduced sensitivity to the lower conductivity sandlayers where hydrocarbon reserves are produced. To address this problem,loggers have turned to using transverse induction logging tools havingmagnetic transmitters and receivers (induction coils) orientedtransversely with respect to the tool longitudinal axis. Suchinstruments for transverse induction well logging has been described inPCT Patent publication WO 98/00733 of Beard et al. and U.S. Pat. No.5,452,761 to Beard et al.; U.S. Pat. No. 5,999,883 to Gupta et al.; andU.S. Pat. No. 5,781,436 to Forgang et al.

In transverse induction logging tools, the response of transversal coilarrays is also determined by an average conductivity; however, therelatively lower conductivity of hydrocarbon-bearing sand layersdominates in this estimation. In general, the volume of shale/sand inthe formation can be determined from gamma-ray or nuclear well loggingmeasurements. Then a combination of the conventional induction loggingtool with transmitters and receivers oriented along the well axis andthe transversal induction logging tool can be used for determining theconductivity of individual shale and sand layers.

One, if not the main, difficulties in interpreting the data acquired bya transversal induction logging tool is associated with vulnerability ofits response to borehole conditions. Among these conditions is thepresence of a conductive well fluid as well as wellbore fluid invasioneffects

In induction logging instruments, the acquired data quality depends onthe formation electromagnetic parameter distribution (conductivity orresistivity) in which the tool induction receivers operate. Thus, in theideal case, the logging tool measures magnetic signals induced by eddycurrents flowing in the formation. Variations in the magnitude and phaseof the eddy currents occurring in response to variations in theformation conductivity are reflected as respective variations in theoutput voltage of receivers. In the conventional induction instrumentsthese receiver induction coil voltages are conditioned and thenprocessed using analog phase sensitive detectors or digitized by digitalto analog converters and then processed with signal processingalgorithms. The processing allows for determining both receiver voltageamplitude and phase with respect to the induction transmitter current ormagnetic field waveform. It has been found convenient for further upholegeophysical interpretation to deliver the processed receiver signal as avector combination of two voltage components: one being in-phase withtransmitter waveform and another out-of-phase, quadrature component.Theoretically, the in-phase coil voltage component amplitude is the moresensitive and noise-free indicator of the formation conductivity.

There are a few hardware margins and software limitations that impact aconventional transversal induction logging tool performance and resultin errors appearing in the acquired data.

The general hardware problem is typically associated with an unavoidableelectrical field that is irradiated by the tool induction transmittersimultaneously with the desirable magnetic field, and it happens inagreement with Maxwell's equations for the time varying field. Thetransmitter electrical field interacts with remaining modules of theinduction logging tool and with the formation; however, this interactiondoes not produce any useful information. Indeed, due to thealways-existing possibility for this field to be coupled directly intothe receiver part of the sensor section through parasitic displacementcurrents, it introduces the noise. When this coupling occurs, theelectrical field develops undesirable electrical potentials at the inputof the receiver signal conditioning, primarily across the induction coilreceiver, and this voltage becomes an additive noise component to thesignal of interest introducing a systematic error to the measurements.

The problem could become even more severe if the induction logging tooloperates in wells containing water-based fluids. The water-based mud hasa significantly higher electrical permittivity compared to the air or tothe oil-based fluid. In the same time, the electrical impedance to theabove mentioned displacement currents can be always considered ascapacitive coupling between the source—the induction transmitter and thepoint of coupling. This circumstance apparently would result in a factthat capacitive coupling and associated systematic errors areenvironment dependant because capacitive impedance will be converse tothe well mud permittivity.

The conventional method in reducing this capacitive coupling in theinduction logging instrument lays in using special electrical (Faraday)shields wrapped around both transmitter and receiver induction coils.These shields are electrically attached to the transmitter analog groundcommon point to fix their own electrical potential and to providereturns of the displacement currents back to their source—transmitterinstead of coupling to any other place in the tool. However, geometryand layout effectiveness of Faraday shields becomes marginal andcontradictory in the high frequency applications where conventionaltransverse induction tools can operate. These limitations occur due tothe attenuation these shields introduce to the magnetic field known inthe art as a shield “skin effect”. The shield design limitations areunavoidable and, therefore, the possibility for the coupling throughdisplacement currents remains.

Another source of hardware errors introduced into the acquired log datais associated electrical potential difference between different toolconductive parts and, in particular, between transmitter and receiverpressure housings if these modules are spaced apart or galvanicallyseparated. These housings cover respective electronic modules andprotect them from exposure to the harsh well environment including highpressure and drilling fluids. Typically, the pressure housing has asolid electrical connection to the common point of the electronic moduleit covers, however, design options with “galvanically” floating housingsalso exist. If for some reasons, mainly imperfections in conventionalinduction tools, the common points of different electronic modules havean electrical potential difference between them, this difference willappear on the pressure housings. It may occur even in a design with“galvanically” floating housings if the instrument operates at the highfrequencies and, in particular, through the capacitive coupling thatthese metal parts might have to the electronic modules encapsulated in aconductive metallic package.

Having different electrical potentials on separate pressure housingswill force the electrical current to flow between them. This currentwould have a conductive nature and high magnitude if the induction toolis immersed in a conductive well fluid and it will be a displacementcurrent of typically much less magnitude for tool operations in a lessconductive or oil-based mud. In both cases this current is time-varying;therefore it produces an associated time varying magnetic field that isenvironmentally dependent and measured by the induction receiver. Forthose who are skilled in the art it should be understood that theundesirable influence of those currents on the log data would besignificantly higher in the conventional transverse induction toolcompared to the instruments having induction coils coaxial with the toollongitudinal axis only. In particular, this is due to the commonlyaccepted overall design geometry of induction logging tools wheretransmitter and receiver sections are axially separated by the mandrel.It can be noticed that employing the induction tool in the loggingstring where it has mechanical and electrical connections (includingtelemetry) with instruments positioned both above and below could alsoresult in the appearance of the above-mentioned currents.

Another source of the housings' potential offsets is the induction tooltransmitter itself. The remaining electrical field that this transmitterirradiates simultaneously with a magnetic field could be different onthe surface of separate pressure housings. Severity of this error alsodepends on Faraday shields' imperfections as described earlier.

There is an additional problem that the potential difference creates inconventional tool layouts having transmitter and receiver electronicmodules spaced apart and using interconnection wires running throughoutthe sensor (mandrel) section. These wires should be electrically andmagnetically shielded from induction receiver coils in the sensorsection. The entire bundle of wires is placed inside of a highlyconductive metal shield that is electrically connected to the commonpoints of separated transmitter and receiver electronic modules. Thisshield's thickness is selected to enable sufficient suppression ofmutual crosstalk between wires and sensor section coils within theentire operational frequency bandwidth and, primarily, at its lower end.In some cases, this shield is a hollow copper pipe, often called as afeed-though pipe, with a relatively thick wall.

However, besides protecting the sensor section transmitter and receivercoils and interconnecting wires from mutual crosstalk, this shieldsimultaneously creates a galvanic path for the currents that could bedriven by pressure housings and/or electronic potential difference, orinduced by the induction transmitter (as discussed in U.S. Pat. No.6,586,939 to Fanini et al, having the same assignee as the presentapplication and the contents of which are incorporated herein byreference). This path apparently exists along the shield's externalsurface and for a given frequency its depth and impedance has beencontrolled by the shield geometry, material conductivity and magneticpermeability. The time varying currents also generate a respectivemagnetic field that crosses induction receiver coils and induces errorvoltages. Unfortunately, these error voltages are also environmentallydependent and their changes cannot be sufficiently calibrated out duringtool manufacturing. The overall analysis of the potential differenceinfluence demonstrates that in the conductive well fluid, galvaniccurrents flowing through the fluid along external surface of theinduction tool would dominate. The superposition and magnitude of thesegalvanic currents strongly depend up on the ambient temperature thatpushes the conventional induction tool performance to furtherdeterioration.

Another source of systematic errors introduced in the log data isdirectly determined by uncertainties in mechanical dimensions ofmulti-component transmitter and receiver coils in the sensor sectionrelated both to their overall dimensions and positions with respect toeach other. Thus, to keep required signal phase relationships,conventional tool designs have primarily relied on the mechanicalstability and electrical properties of advanced ceramics and plasticmaterials to build the mandrel. However, even slight physical assemblydeviations in the coil wires position and non-uniform coil form materialtemperature dependencies might destroy a factory pre-set compensation ofthe transmitter primary magnetic field coupled in the receiver coil(bucking) during well logging, and create non-recoverable errors due tomechanical displacement or imperfections.

U.S. Pat. No. 6,734,675 and U.S. Pat. No. 6,586,939 to Fanini et al.,both having the same assignee as the present application and thecontents of which are incorporated herein by reference, address some ofthe issues present in the calibration and use of multicomponentinduction logging tools. Fanini '939 discloses a transverse inductionlogging tool having a transmitter and receiver for downhole sampling offormation properties, the tool having a symmetrical shielded split-coiltransmitter coil and a bucking coil interposed between the splittransmitter coils to reduce coupling of the transmitter time varyingmagnetic field into the receiver. The tool provides symmetricalshielding of the coils and grounding at either the transmitter orreceiver end only to reduce coupling of induced currents into thereceived signal. The tool provides an insulator between receiverelectronics and the conductive receiver housing having contact withconductive wellbore fluid, to reduce parasitic current flowing in a loopformed by the upper housing, feed through pipe, lower housing andwellbore fluid adjacent the probe housing or mandrel. An internalverification loop is provided to track changes in transmitter current inthe real and quadrature component of the received data signal.

Fanini '675 discloses a transverse induction logging tool having atransmitter and receiver for downhole sampling of formation properties,the tool having a symmetrical shielded split-coil transmitter coil and abucking coil interposed between the split transmitter coils to reducecoupling of the transmitter time varying magnetic field into thereceiver. The tool provides symmetrical shielding of the coils andgrounding at either the transmitter or receiver end only to reducecoupling of induced currents into the received signal. The tool providesan insulator between receiver electronics and the conductive receiverhousing having contact with conductive wellbore fluid, to reduceparasitic current flowing in a loop formed by the upper housing, feedthrough pipe, lower housing and wellbore fluid adjacent the probehousing or mandrel. An internal verification loop is provided to trackchanges in transmitter current in the real and quadrature component ofthe received data signal

SUMMARY OF THE INVENTION

One embodiment of the invention is a method of preparing amulticomponent induction tool having a plurality of transmitter coilsand a plurality of receiver coils. The method includes positioning thelogging tool in a calibration area substantially free from componentscapable of interfering with magnetic and electric fields produced by thetool. A first conductive housing of the tool is coupled with a secondconductive housing of the tool through a borehole conductivity simulatorhaving an impedance similar to that of a borehole environment. A firstcoil of the plurality of transmitter coils is activated and the signalin a first coil of the plurality of receiver coils is measured. Thefirst coil of the plurality of transmitter coils is moved relative to aconductive feed-through pipe between the first housing and a secondhousing to reduce the magnitude of the signal. The first coil of theplurality of receive accordance is moved relative to the feed-throughpipe until the magnitude of the signal is substantially equal to zero.That method may further include positioning the first coil of theplurality of receiver coils in an eccentered position in the loggingtool prior to making the measurement. The method may further compriseorienting the logging tool with its longitudinal axis substantiallyparallel to the ground. The first coil of the plurality of transmittercoils may have an axis that is substantially parallel to a longitudinalaxis of the tool or substantially orthogonal to a longitudinal axis ofthe tool. The first coil for the plurality of receiver coils may have anaxis that is substantially parallel to a longitudinal axis of the toolor substantially orthogonal to a longitudinal axis of the tool. Themethod may further include rotating the tool about a longitudinal axisof the tool, activating a second coil of the plurality of transmittercoils and measuring an additional signal in a second coil of theplurality of receiver coils, and moving the second coil of the pluralityof transmitter coils with respect to the feed-through pipe to reduce themagnitude of the additional signal. The method may further includemagnetically coupling the tool to a calibrator, activating the firstcoil of the plurality of transmitter coils, and determining from asignal received at a specific coil of the plurality of receiver coils atransfer function between the specific coil and a first coil of thegravity of transmitter coils. The tool may be positioned inside thecalibrator.

Another embodiment of the invention is an apparatus for evaluatingperformance of the multicomponent induction logging tool having aplurality of transmitter coils and a plurality of receiver coils. Thetool is positioned in a calibration area substantially free fromcomponents capable of interfering with magnetic and electric fieldsproduced by the tool. The apparatus includes a borehole conductivitysimulator having an impedance similar to that of a borehole environment,the borehole conductivity simulator coupling a first housing of the toolwith a second housing of the tool. The apparatus includes a processorconfigured to activate a first coil of the plurality of transmittercoils. The apparatus further includes a first coil of the plurality ofreceiver coils configured to provide a signal responsive to theactivation of the first transmitter coil. The apparatus includes adevice configured to move the first coil of the plurality of transmittercoils relative to the first coil of the plurality of receiver coils toreduce the magnitude of signal, and move the first coil of the pluralityof receiver coils relative to the conductive feed-through pipe until themagnitude of the signal is substantially zero. The device may further beconfigured to position the first coil of the plurality of receiver coilsin an eccentered position in the logging tool. The first coil of theplurality of transmitter coils may have an axis that is substantiallyparallel to the longitudinal axis of the tool or substantially ororganelle to a longitudinal axis of the tool. The first coil of theplurality of receiver coils may have an axis that is substantiallyparallel to a longitudinal axis of the tool or substantially orthogonalto a longitudinal axis of the tool. The apparatus may further include acalibrator wherein the logging tool is magnetically coupled with acalibrator and the processor is further configured to determine from thesignal a transfer function between the first coil of the plurality oftransmitters and the first coil of the plurality of receivers.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is best understood with reference to theaccompanying figures in which like numerals refer to like elements andin which:

FIG. 1 (prior art) shows schematically a wellbore extending into alaminated earth formation, into which wellbore an induction logging toolas used according to the invention has been lowered;

FIG. 2A (prior art) illustrates a conventional resistivity measurementin the vertical direction;

FIG. 2B (prior art) illustrates a resistivity measurement in thehorizontal direction;

FIG. 3 is an overall flow chart of the procedures of the presentinvention;

FIG. 4 illustrates a borehole conductivity simulator (BCS) used in thepresent invention;

FIG. 5 illustrates an assembly for calibrating of transverse arrays in alogging tool;

FIG. 6 illustrates an assembly for calibrating longitudinal arrays in alogging tool;

FIGS. 7-8 illustrate assemblies for calibrating XY cross-componentarrays; and

FIGS. 9-10 illustrate assemblies for calibrating XZ cross-componentarrays.

DETAILED DESCRIPTION OF THE INVENTION

The instrument structure provided by the present invention enablesincreased stability and accuracy in an induction wellbore logging tooland its operational capabilities, which, in turn, results in betterquality and utility of wellbore data acquired during logging. Thefeatures of the present invention are applicable to improve thestructure of a majority of known induction tools.

The invention will now be described in more detail and by way of examplewith reference to the accompanying drawings. FIG. 1 schematically showsa wellbore 1 extending into a laminated earth formation, into whichwellbore an induction logging tool as used according to the presentinvention has been lowered. The wellbore in FIG. 1 extends into an earthformation which includes a hydrocarbon-bearing sand layer 3 locatedbetween an upper shale layer 5 and a higher conductivity than thehydrocarbon bearing sand layer 3. An induction logging tool 9 used inthe practice of the invention has been lowered into the wellbore 1 via awire line 11 extending through a blowout preventor 13 (shownschematically) located at the earth surface 15. The surface equipment 22includes an electric power supply to provide electric power to the setof coils 18 and a signal processor to receive and process electricsignals from the receiver coils 19. Alternatively, the power supplyand/or signal processors are located in the logging tool.

The relative orientation of the wellbore 1 and the logging tool 9 withrespect to the layers 3, 5, 7 is determined by two angles, one of whichθ as shown in the FIG. 1. For determination of these angles see, forexample, U.S. Pat. No. 5,999,883 to Gupta, et al. The logging tool 9 isprovided with a set of transmitter coils 18 and a set of receiver coils19, each set of coils 18, 19 being connected to surface equipment 22 viasuitable conductors (not shown) extending along the wire line 11.

The relative orientation of the wellbore 1 and the logging tool 9 withrespect to the layers 3, 5, 7 is determined by two angles, one of whichθ as shown in the FIG. 1. For determination of these angles see, forexample, U.S. Pat. No. 5,999,883 to Gupta, et al. The logging tool 9 isprovided with a set of transmitter coils 18 and a set of receiver coils19, each set of coils 18, 19 being connected to surface equipment 22 viasuitable conductors (not shown) extending along the wire line 11.

Each set of coils 18 and 19 includes three coils (not shown), which arearranged such that the set has three magnetic dipole moments in mutuallyorthogonal directions, that is, in x, y and z directions. The three-coiltransmitter coil set transmits magnetic fields T_(x), T_(y) and T_(z);the receiver coils measure induced signal from main directions R_(x),R_(y) and R_(z) as well as the cross components, R_(xy), R_(xz) andR_(zy). Thus, coil set 18 has magnetic dipole moments 26 a, 26 b, 26 c,and coil set 19 has magnetic dipole moments 28 a, 28 b, 28 c. In oneembodiment the transmitter coil set 18 is electrically isolated from thereceiver coil set 19. The coils with magnetic dipole moments 26 a and 28a are transverse coils, that is they are oriented so that the magneticdipole moments are oriented perpendicular to the wellbore axis, wherebythe direction of magnetic dipole moment 28 a is opposite to thedirection of magnetic dipole moment 26 a. Furthermore the sets of coils18 and 19 are positioned substantially along the longitudinal axis ofthe logging tool 9.

As shown in FIG. 2A, conventional induction logging tools provide asingle transmitter and receiver coil that measure resistivity in thehorizontal direction. In the mode shown in FIG. 2A, the resistivities ofadjacent high resistivity sand and low resistivity shale layers appearin parallel, thus the resistivity measurement is dominated by lowresistivity shale. As shown in FIGS. 1 and 2B, in the present inventiona transverse coil is added to measure resistivity in the verticaldirection. In the vertical direction, the resistivity of the highlyresistive sand and low resistivity shale are appear in series and thusthe vertical series resistivity measurement is dominated by theresistivity of the highly resistive sand.

For ease of reference, normal operation of the tool 9, as shown in FIGS.1 and 2B, will be described hereinafter only for the coils having dipolemoments in the x-direction, i.e. dipole moments 26 a and 28 a. Duringnormal operation an alternating current of a frequency f₁ has beendriven by the tool electronics (not shown) connected to the coil 26which, in turn, is supplied by the electric power supply of surfaceequipment 22 to transmitter coil set 18 so that a magnetic field withmagnetic dipole moment 26 a is induced in the formation. In analternative embodiment, the frequency is swept through a range f₁through f₂. This magnetic field extends into the sand layer 3 andinduces a number of local eddy currents in the sand layer 3. Themagnitude of the local eddy currents is dependent upon their locationrelative to the transmitter coil set 18, the conductivity of the earthformation at each location, and the frequency at which the transmittercoil set 18 is operating. In principle, the local eddy currents act as asource inducing new currents, which again induce further new currents,and so on. The currents induced into the sand layer 3 induces a responsemagnetic field in the formation, which is not in phase with thetransmitted magnetic field, but which induces a response signal inreceiver coil set 19. The magnitude of the current induced in the sandlayer 3 depends on the conductivity of the sand layer 3, the magnitudeof the response current in receiver coil set 19. The magnitude alsodepends on the conductivity and thereby provides an indication of theconductivity of the sand layer 3. However, the magnetic field generatedby transmitter coil set 18 not only extends into sand layer 3, but alsoin the wellbore fluid and in the shale layers 5 and 7 so that currentsin the wellbore fluid and the shale layers 5 and 7 are induced.

The overall procedures of the present invention used to ensure properfunctioning of a deployed multicomponent induction logging tool issummarized in FIG. 3. Calibration of the instrument's arrays is done,particularly estimating its transfer coefficient 101. Subsequently, afinal verification of the tuning and calibration consistency isperformed 103. This is followed by a verification of isolatorsufficiency 105 for preventing an axial current flow between the tool'stop and bottom housings/electronics through the feed-through pipe andconductors while logging in the boreholes filled with conductive mud.

In further detail, the fully made tool is placed in calibration areawhich has a small number of external parts that could interfere withmagnetic and electric fields produced or received by the tool and thusaffect tool readings (machinery, measurement tools, etc.). For example,positioning the tool at approximately 15 ft (4.6 m) above the groundtypically reduces the tool environmental reading to a value less thanabout 10 mS/m. The tool is positioned parallel to the Earth with thearray to be adjusted pointing normal to the ground.

FIG. 4 illustrates the BCS, comprising an assembly of conductor 401 andresistor 410, which electrically couples top housing 405 and bottomhousing 404. A closed circuit is thus created from bottom housing 404through resistor 410 through top housing 405 through a feed-through piperunning from bottom housing to top housing through mandrel 408. Thevalue of resistor 410 can be configured to be approximately equal to atotal conductivity (or resistivity) value between top and bottomhousings which the tool would experience inside a borehole according toits specifications. A resistance value of approximately 20 Ohms istypically chosen.

In this arrangement the tool becomes very sensitive to the axial currentthat could be induced by the array transmitter in the following loop:“top housing—conductive feed-through pipe—bottom housing—BCS”. Themagnitude of the current will be proportional to the array coilsdisplacement from their longitudinal alignment (almost true for smalldisplacements ˜1/d) and simulator resistor value.

To balance the array its transmitter coil may be moved in the planeparallel to the ground. This coil movement is performed until anabsolute minimum in the receiver reading is reached. In one embodimentof the invention, the receiver coil is positioned off-center relative tothe tool. At this position, the receiver signal is particularlysensitive to misalignment of the transmitter. This makes it easier todetermine the minimum. Upon adjustment the transmitter coil frame isfixed inside the mandrel. This could be accomplished with the sets ofnon-conductive screws and/or with epoxy; however, alternative meanscould be applied, as well. Shorting the isolator between the upperhousing and the mandrel is done to significantly increase the magnitudeof the axial current in this test procedure and, therefore, increaseaccuracy of balancing. A similar positioning may be done in the verticaldirection. As discussed below, the tool is more sensitive tomis-positioning in the vertical direction than in the horizontaldirection. Suitable positioning screws may be provided in the loggingtool to accomplish this movement.

Following the positioning of the transmitter coil, the receiver coil ismoved to a position where the signal and the receiver coil is zero. Whenthis is done, the particular transmitter and receiver are properlybalanced. The description above has been made with respect to movementof the coils relative to each other. It is to be understood that whenthese movements are made, the coils are also being moved relative to thefeed-through pipe.

After the first horizontal array has been tuned the tool is rotatedabout its axis and similar procedure has been performed with nexthorizontal array. Generally, the instrument might have a plurality oftransverse and tilted arrays so that similar tuning could be developedfor each sensor. After balance of all arrays has been completed, thetool isolation short is removed and mandrel is covered with thenon-conductive pressure sleeve.

Calibration of transfer coefficient is done after the instrument ispositioned in the low conductive calibration environment and insertedinside the calibrator. The calibration principle lies in introducing acertain dissipative load through magnetic coupling for calibrating arrayso that its signal readings are identical to the values to be read whilelogging a homogeneous formation with finite conductivity. This is donewith use of a calibrator whose electromagnetic parameters and couplingwith the tool are precisely known. Using the calibrator, tool loading isachieved by the connecting certain impedance to the terminal ofnormally-open calibrator loop. Thus, the open loop presents aninfinitely resistive formation. Conversely, by shorting, almostinfinitely conductive formation is presented. Therefore, any value ofthe formation conductivity corresponds to its unique value of thecalibration loop load.

Acquiring the calibration signal is typically done in the mode“calibration load connected-disconnected”. This difference in the toolreading indicates on how much the tool output voltage swings when theformation conductivity changes from 0 to the calibrated value. Toperform calibration the tool array may be oriented normal to the groundas this leads to more consistency in measurements and apparently makeits transversal arrays less sensitive to any residual noise currentsthat maybe circulating on the Earth surface in place of measurement(machinery, radio-stations, etc.).

After the tool transfer coefficient has been determined, the toolreadings while the calibrator loop is not loaded reflect environmentalconductivity and, in particular, ground conductivity. This data has tobe known and stored for further processing.

The last step in calibration is verification of the tool symmetry andimmunity to axial currents. The overall tool symmetry assumes that thesame array reads the same values of the “ground” or environmentalconductivity while its measurement direction points to ground or fromthe ground. For these purposes the tool is rotated around itslongitudinal axis on 180°. Absence of such a “direction sensitivity”would indicate normal tool functioning and ensure respective symmetrywhile operating in the well bore.

For verification of the suppressing axial currents—a modified BCS testmay be run with the short removed in the feed-through. Thus, connectingand disconnecting the BCS to the tool should result in absolute minimaldifference in readings that would indicate for proper operation in thewell without formation-dependable offset in the tool data. This modifiedBCS test could be run as described, or, to reduce calibration time,performed right after the transfer coefficient is determined.

Turning now to FIG. 5, one arrangement of the alignment loop isdiscussed. Shown therein is an alignment loop 501 surrounding an arraycharacterized by the transmitter coil 504 directed along an X direction(T_(x)) and the receiver coil 508 directed along the X direction(R_(x)). Bucking coil B_(x) 506 is also shown. This array is denoted asXX, using a nomenclature in which the first letter signifies theorientation direction of the transmitter coil and the last lettersignifies the orientation direction of the receiver coil. Thisnomenclature is generally used herein. The XX and YY arrays in themulti-component tool are ideally aligned at 90° from each other. Whenthis alignment is not met, the response of the cross components (XY, YX)are affected by part of the reading of the related main component. Thealignment measuring method of the present invention is based onanalyzing the output of the cross-component system when the tool isrotated inside of an alignment loop.

The alignment loop 501 is a stationary loop, lying so that thelongitudinal axis of the loop and the longitudinal axis of thewell-logging tool are substantially aligned. Its dimensions are such asto obtain substantial inductive coupling with the transmitter as well aswith the receiver of both XX and YY arrays. The long “box” calibrator ofFIG. 4 is used to performed calibration of the horizontal arrays. Adetailed analysis of the signals is given later in this document.

FIG. 6 illustrates a loop alignment assembly usable for aligning ZZarrays in a testing device. Transmitter TZ 601, bucking coil BZ 603 andreceiver RZ 605 are disposed along the feed-through pipe 615 and have acommon longitudinal axis. Alignment loop 610 is substantially coaxialwith receiver RZ 605 and substantially centered on RZ.

Cross component array calibration is discussed next. FIG. 7 illustratesan embodiment for calibration of an XY array using a calibration box.Transmitter 701 and bucking coil 703 are disposed along the feed-throughpipe oriented to produce a magnetic moment in an X-direction. Receiver705 is disposed along the same feed-through pipe having an orientationso as to receive components of a magnetic moment in a is disposed alongthe same feed-through pipe having an orientation so as to receivecomponents of a magnetic moment in Y-direction. The alignment box 710 isdisposed at an angle of 45° so as to be oriented halfway between theX-direction and the Y-direction.

FIG. 8 illustrated an alternate embodiment for aligning an XY array.Alignment box 815 is located at the TX 801, and alignment box 810 ispositioned at the RXY cross-component receiver 805. Both alignment boxesare oriented along the same direction as their respectivetransmitter/receiver. A wire 820 electrically couples alignment box 810and alignment box 815. (in this configuration box 815 receives signalfrom transmitter coil, the voltage induced across its winding producescurrent flowing through winding of both boxes and load impedance. Whilegoing though winding of 810 this current generates filed that is pickedup by cross-component receiver)

FIG. 9 illustrates an assembly for orienting of the XZ cross-componentarray. Transmitter TX 901 and bucking coil BX 903 are disposed along thefeed-through pipe oriented so as to produce a magnetic moment along anX-direction. The receiver RZ 905 is disposed along the feed-through pipeand oriented so as to be receptive to Z-components of magnetic moments.The alignment box 920 can be positioned centrally between mainX-transmitter 901 and Z-cross-component receiver 905 and tilted 45° withrespect to the tool longitudinal axis 910. The assembly of FIG. 8displays small signals during XZ array calibration. This signal tends todisplay a high sensitivity to the angle.

FIG. 10 illustrates an alternate embodiment for aligning the XZcross-component array. Transmitter TX 1001 and bucking coil BX 1003 aredisposed along the feed-through pipe oriented so as to produce amagnetic moment along an X-direction. The receiver RZ 1005 is disposedalong the feed-through pipe and oriented so as to be receptive toZ-components of magnetic moments. Alignment box 1010 is centered ontransmitter TX 1001, and alignment loop 1015 is coaxial with receiver RZ1005. A wire 1020 electrically couples alignment box 1010 and alignmentloop 1015. In contrast to the assembly of FIG. 0, calibration using twoalignment devices displays a large signal for the XZ array calibration.

We next discuss in detail the use of the alignment box for establishingthe coil orientation. When examining a cross-component array, the XY orYX response obtained by rotating the tool inside of the alignment loophas a zero-crossing each time that either a transmitter or a receivercoil is perpendicular to the plane of the loop. Whichever coil(transmitter or receiver) is substantially aligned with the loop(enclosed in the same plane) experiences a maximum coupling with thealignment loop. When the position of the aligned coil is varied aroundthe point of alignment with the alignment loop, the coupling responsebetween them undergoes a slow change corresponding to the variation. Thenon-aligned coil experiences a minimum coupling with the alignment loop.When the position of the non-aligned coil is varied around this point ofminimal coupling, the coupling experiences an abrupt change. Thecoupling becomes zero when the non-aligned coil achievesperpendicularity with the alignment loop. A practitioner in the artwould recognize that the zero-crossings of the coupling response aresignificantly affected by the coil that is at right angle to thealignment loop, regardless of whether the perpendicular coil is areceiver or a transmitter. The substantially aligned coil plays littleor no role in the production of a zero-crossing. The angle betweensuccessive zero crossings thereby represents an alignment angle betweenthe two related coils.

Mathematically, the inductive coupling between two coils resembles acosine function of the angle between them. Thus, the coupling responsesystem of coils made by an aligned system of cross components and analignment loop is given by the following expression:

$\begin{matrix}{{R(\varphi)} = {K \cdot {\cos (\varphi)} \cdot {{\cos \left( {\varphi - \frac{\pi}{2}} \right)}.}}} & (1)\end{matrix}$

Applying trigonometric identities, Eq. (1) can be simplified to

R(φ)=K·cos(φ)·sin(φ),  (2).

and since

$\begin{matrix}{{{{\sin (\varphi)} \cdot {\cos (\varphi)}} = {\frac{1}{2}{\sin \left( {2 \cdot \varphi} \right)}}},} & (3)\end{matrix}$

it follows that

$\begin{matrix}{{R(\varphi)} = {K \cdot \frac{1}{2} \cdot {{\sin \left( {2 \cdot \varphi} \right)}.}}} & (4)\end{matrix}$

Eqn. (4) illustrates that there are two cycles of variation for eachcycle of tool rotation.

By considering a misalignment angle β between transmitter and receiver,the response function can now be expressed as

$\begin{matrix}{{{R\left( {\varphi,\beta} \right)} = {K \cdot {\cos (\varphi)} \cdot {\cos \left( {\varphi - \frac{\pi}{2} + \beta} \right)}}},} & (5)\end{matrix}$

where each cosine function characterizes the response of the individualcross component coils. It is easy to see that

R(φ,β)=0  (6),

when

$\begin{matrix}{{{\varphi = {n \cdot \frac{\pi}{2}}}{{or}\mspace{14mu} {when}}{\varphi - \frac{\pi}{2} + \beta} = {{{n \cdot \frac{\pi}{2}}\mspace{14mu} {with}\mspace{14mu} n} = {\pm 1}}},2,3,\ldots} & {{Eq}.\mspace{14mu} (7)}\end{matrix}$

According to eqn. (7), the angle between successive zero-crossingsrepresents the alignment angle among the cross component coils. Anintuitive graphical approach can therefore be used to measure themisalignment angle between transmitter and receiver.

Alternatively, the misalignment angle can be obtained simply by using atrigonometric regression function to analyze the response of the system.Applying trigonometric identities to Eqn. (5), the response of themisaligned system can be written as

$\begin{matrix}{{{R\left( {\varphi,\beta} \right)} = {{K \cdot {\cos (\varphi)} \cdot {\sin (\varphi)} \cdot {\cos (\beta)}} + {K \cdot {\cos^{2}(\varphi)} \cdot {\sin (\beta)}}}}{{R\left( {\varphi,\beta} \right)} = {{K \cdot \frac{1}{2} \cdot {\sin \left( {2 \cdot \varphi} \right)} \cdot {\cos (\beta)}} + {K \cdot {\cos^{2}(\varphi)} \cdot {\sin (\beta)}}}}{{R\left( {\varphi,\beta} \right)} = {{\frac{K}{2} \cdot {\sin \left( {{2 \cdot \varphi} + \beta} \right)}} + {\frac{K}{2} \cdot {\sin (\beta)}}}}} & (8)\end{matrix}$

The last expression in eqn. (8) indicates that a graphicalrepresentation of the coupling response of the misaligned crosscomponent system resembles a sinusoidal function. The period of thissinusoid equals 180° and has offsets on both the abscissa and theordinate. The offset on the abscissa is β, and the offset on theordinate is (K/2)sin(β). Also, the coupling response is of the form Asin(x+B)+C, where A=K/2, B=β and C=(K/2)(sin(β). The coefficient βobtained with such fitting represents the misalignment angle. The crosscomponent response can thus be fit to this curve.

The sensitivity to possible displacement along the tool's longitudinalaxis or vertically can be analyzed by changes in the productM=M_(T-C)M_(C-R), where M_(T-C) is the mutual inductance between thetransmitter and the alignment coils, and M_(R-C) is the mutualinductance between the alignment and the receiver coils. Table 1illustrates mutual inductances that result from misalignment ordisplacement of an alignment coil in the horizontal direction(longitudinally). There is in general a flexibility of 1″ withoutsubstantially affecting the induction response.

TABLE 1 Calibrator Displacement M_(T-C) M_(C-R) M [inch] [microHenry][microHenry] [μH]² 4 13.348 16.700 222.912 2 13.409 13.580 182.094 113.443 13.521 181.763 ¾ 13.452 13.510 181.736 ½ 13.461 13.499 181.710 013.479 13.479 181.683 −½  13.499 13.461 181.710 −¾  13.510 13.452181.736 −1  13.521 13.443 181.763 −2  13.580 13.409 182.094 −4  16.70013.348 222.912Table 2 shows the effects of misalignment in the vertical direction. Amisalignment exceeding 5/16″ produces an error greater than 0.22%. Thusvertical misalignment has a greater effect on induction response thanhorizontal misalignment.

TABLE 2 Calibrator Displacement M_(T-C) M_(C-R) M [inch] [microHenry][microHenry] [μH]² % 0 13.479 13.479 181.683 0 3/16 13.474 13.474181.549 0.074 5/16 13.464 13.464 181.279 0.22 7/16 13.449 13.449 180.8760.44

To properly position the arrays, the transmitter coil of one array ismoved in the direction normal to the ground. This coil movement isperformed until an absolute minimum in the coupling response isdetermined. Upon adjustment, the transmitter coil frame is fixed insidethe mandrel. After the first horizontal array has been tuned, the toolis rotated on its axis and a similar procedure is performed with theother horizontal array. Generally, similar tuning can be developed foran instrument having a plurality of transverse and tilted arrays. Afterbalance of all arrays has been achieved, the tool isolation short isremoved and mandrel is covered with the non-conductive pressure sleeveto protect induction coils from being directly exposed to boreholefluids.

A final verification of the coil balancing and calibration consistencyis made. Calibration of a transfer coefficient is performed once theinstrument is inserted inside the calibrator in the low conductivecalibration environment. A magnetic load is introduced suitable forcalibrating array, so that its signal readings are identical to thevalues to be read while logging a homogeneous formation. The magneticload is introduced using the above-referenced calibrator using knownelectromagnetic parameters and coupling parameters. The tool loading canbe achieved by connecting selected impedance to the terminal of anormally-open calibrator loop. Thus, the open loop represents aninfinitely resistive formation. Once shorted, the closed loop representsan almost infinitely conductive formation (limited only by internalimpedance of the wires of the calibrator loop). Therefore, a calibrationloop load can be chosen effectively representing a given formationconductivity values.

Implicit in the control and processing of the data is the use of acomputer program on a suitable machine readable medium that enables theprocessor to perform the control and processing. The machine readablemedium may include ROMs, EPROMs, EEPROMs, Flash Memories and Opticaldisks.

While the foregoing disclosure is directed to the preferred embodimentsof the invention, various modifications will be apparent to thoseskilled in the art. It is intended that all variations within the scopeand spirit of the appended claims be embraced by the foregoingdisclosure.

1. A method of preparing a multicomponent induction logging tool havinga plurality of transmitter coils and a plurality of receiver coils, themethod comprising: (a) positioning the logging tool in a calibrationarea substantially free from components capable of interfering withmagnetic and electric fields produced by the said tool; (b) coupling afirst conductive housing of the tool with a second conductive housing ofthe tool through a borehole conductivity simulator (BCS) having animpedance similar to that of a borehole environment; (c) activating afirst coil of the plurality of transmitter coils and measuring a signalin a first coil of the plurality of receiver coils; (d) moving the firstcoil of the plurality of transmitter coils relative to a conductivefeed-through pipe between the first housing and the second housing toreduce a magnitude of the signal; and (e) moving the first coil of theplurality of receiver coils relative to the feed-through pipe until themagnitude of the signal is substantially equal to zero.
 2. The method ofclaim 1 further comprising positioning the first coil of the pluralityof receiver coils in an eccentered position in the logging tool prior tostep (d).
 3. The method of claim 1 further comprising orienting thelogging tool with its longitudinal axis substantially parallel to theground.
 4. The method of claim 1 wherein the first coil of the pluralityof transmitter coils has an axis that is one of (i) substantiallyparallel to a longitudinal axis of the tool, and (ii) substantiallyorthogonal to a longitudinal axis of the tool.
 5. The method of claim 1wherein the first coil of the plurality of receiver coils has an axisthat is one of (i) substantially parallel to a longitudinal axis of thetool, and (ii) substantially orthogonal to a longitudinal axis of thetool.
 6. The method of claim 1 further comprising: (i) rotating the toolabout a longitudinal axis of the tool; (ii) activating a second coil ofthe plurality of transmitter coils and measuring an additional signal ina second coil of the plurality of receiver coils; and (iii) moving thesecond coil of the plurality of transmitter coils with respect to thefeed-through pipe to reduce a magnitude of the additional signal.
 7. Themethod of claim 1 further comprising: (i) magnetically coupling the toolto a calibrator; (ii) activating the first coil of the plurality oftransmitter coils; (iii) determining from a signal received at aspecific coil of the plurality of receiver coils a transfer functionbetween the specific coil and the first coil of the plurality oftransmitter coils.
 8. The method of claim 1 wherein the tool ispositioned inside the calibrator.
 9. The method of claim 1 wherein themoving is in a direction selected from (i) substantially parallel to alongitudinal axis of the tool, and (ii) substantially orthogonal to alongitudinal axis of the tool.
 10. An apparatus for evaluatingperformance of a multicomponent induction logging tool having aplurality of transmitter coils and a plurality of receiver coils, thetool being positioned in a calibration area substantially free fromcomponents capable of interfering with magnetic and electric fieldsproduced by the said tool, the apparatus comprising: (a) a boreholeconductivity simulator (BCS) having an impedance similar to that of aborehole environment the BCS coupling a first housing of the tool with asecond housing of the tool; (b) a processor configured to activate afirst coil of the plurality of transmitter coils; (c) a first coil ofthe plurality of receiver coils configured to provide a signalresponsive to the activation of the first coil; and (d) a deviceconfigured to: (A) move the first coil of the plurality of transmittercoils relative to the first coil of the plurality of receiver coils toreduce a magnitude of the signal; and (B) move the first coil of theplurality of receiver coils relative to the conductive feed-through pipeuntil the magnitude of the signal is substantially zero.
 11. The methodof claim 10 wherein the device is further configured to position thefirst coil of the plurality of receiver coils in an eccentered positionin the logging tool.
 12. The apparatus of claim 10 wherein the firstcoil of the plurality of transmitters has an axis that is one of (i)substantially parallel to a longitudinal axis of the tool, and (ii)substantially orthogonal to a longitudinal axis of the tool.
 13. Theapparatus of claim 10 wherein the first coil of the plurality ofreceivers has an axis that is one of (i) substantially parallel to alongitudinal axis of the tool, and (ii) substantially orthogonal to alongitudinal axis of the tool.
 14. The apparatus of claim 10 furthercomprising a calibrator and wherein: (i) the logging tool ismagnetically coupled with the calibrator, and (ii) the processor isfurther configured to determine from the signal a transfer functionbetween the first coil of the plurality of transmitters and the firstcoil of the plurality of receivers.
 15. The apparatus of claim 10wherein the logging tool is positioned within the calibrator.
 16. Theapparatus of claim 9 wherein the device is configured to producemovement in a direction selected from (i) substantially parallel to alongitudinal axis of the tool, and (ii) substantially orthogonal to alongitudinal axis of the tool.